This invention relates to spool valves and, more specifically, to a spool valve provided with means whereby the action of flow forces is generally constant and/or balanced for any degree of opening of the valve.
The existence of flow forces in spool valves and their effects has long been recognized. Depending upon the particular valve construction, the flow pattern therethrough, and the pressures involved, flow forces may tend to oppose an operator opening the valve or act in concert therewith at widely varying values dependent upon spool travel. In many instances, for one position of the spool, the flow force may oppose the operator while in another position of the spool, it may aid the operator.
Consequently, precise control of the flow of fluid may be difficult to achieve to the desired degree because spool position will not be solely determined by a control force provided by an operator for the valve. For when the flow force is assisting the operator force, the total force operating on the spool will be greater than that provided by the operator alone and the total force may vary according to spool position. The same is true when the flow forces are acting in opposition to the operator force and, of course, when flow forces can act both in concert and in opposition to the operator force, dependent upon spool travel, accurate placement of the spool through operator force at all positions along the path of travel of the spool becomes exceedingly difficult to attain. Therefore, there is a real need for a spool valve wherein flow forces are substantially balanced and/or constant throughout the operating parameters of the valve.